1. Field of the Invention
This invention relates to implantable medical devices and, more particularly, to a pacemaker output circuit for developing stimulating pulses to be coupled to the heart in response to logic level control signals.
2. Description of the Prior Art
Implantable medical devices for the therapeutic stimulation of the heart are well known in the art from U.S. Pat. No. 3,478,746, issued to Wilson Greatbatch, which discloses a demand pacemaker. This form of pacer stimulates cardiac activity in the absence of naturally occurring spontaneous cardiac activity. This operational mode requires a sense amplifier for detecting the naturally occurring deplorizations. Since the stimulating pulses and detected depolarizations are supplied through the same lead system to the cardiac tissue, the output stage of the pacer is electrically in parallel with the sense amplifier input resulting in an undesirable reduction in the input sense amplifier impedance.
Modern versions of the demand pacemaker are powered by lithium primary batteries having voltages in the range of 2 to 3 volts, which is substantially below the nominal voltage of earlier mercury-zinc powered pacemakers. As a consequence, it has been common to incorporate a voltage doubler into the output circuitry to provide higher amplitude stimulation of the cardiac tissue. In prior art pacemakers, this is done by a voltage doubler which charges a capacitor to the battery voltage and then switches this charged capacitor into series with the battery, while the combination of battery and capacitor is coupled to the heart.
This prior art configuration requires a relatively large capacitor (10 microfarad) placed in the output circuitry. This capacitor occupies a substantial volume on the hybrid substrate and poses reliability problems for long-term pacer implants.